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.\" Copyright (c) 2009 Michael Kerrisk, <mtk.manpages@gmail.com>
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.\" SPDX-License-Identifier: Linux-man-pages-copyleft
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.TH pthread_setconcurrency 3 2022-12-15 "Linux man-pages 6.03"
.SH NAME
pthread_setconcurrency, pthread_getconcurrency \- set/get
the concurrency level
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.PP
.BI "int pthread_setconcurrency(int " new_level );
.BI "int pthread_getconcurrency(" void );
.fi
.SH DESCRIPTION
The
.BR pthread_setconcurrency ()
function informs the implementation of the application's
desired concurrency level, specified in
.IR new_level .
The implementation takes this only as a hint:
POSIX.1 does not specify the level of concurrency that
should be provided as a result of calling
.BR pthread_setconcurrency ().
.PP
Specifying
.I new_level
as 0 instructs the implementation to manage the concurrency level
as it deems appropriate.
.PP
.BR pthread_getconcurrency ()
returns the current value of the concurrency level for this process.
.SH RETURN VALUE
On success,
.BR pthread_setconcurrency ()
returns 0;
on error, it returns a nonzero error number.
.PP
.BR pthread_getconcurrency ()
always succeeds, returning the concurrency level set by a previous call to
.BR pthread_setconcurrency (),
or 0, if
.BR pthread_setconcurrency ()
has not previously been called.
.SH ERRORS
.BR pthread_setconcurrency ()
can fail with the following error:
.TP
.B EINVAL
.I new_level
is negative.
.PP
POSIX.1 also documents an
.B EAGAIN
error ("the value specified by
.I new_level
would cause a system resource to be exceeded").
.SH VERSIONS
These functions are available since glibc 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR pthread_setconcurrency (),
.BR pthread_getconcurrency ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
The default concurrency level is 0.
.PP
Concurrency levels are meaningful only for M:N threading implementations,
where at any moment a subset of a process's set of user-level threads
may be bound to a smaller number of kernel-scheduling entities.
Setting the concurrency level allows the application to
give the system a hint as to the number of kernel-scheduling entities
that should be provided for efficient execution of the application.
.PP
Both LinuxThreads and NPTL are 1:1 threading implementations,
so setting the concurrency level has no meaning.
In other words,
on Linux these functions merely exist for compatibility with other systems,
and they have no effect on the execution of a program.
.SH SEE ALSO
.BR pthread_attr_setscope (3),
.BR pthreads (7)
